1. Field of the Invention
The present invention relates to a high-frequency amplifier suited to, for example, a mobile communication unit or satellite communication unit.
2. Description of the Related Art
For a mobile communication unit such as a portable telephone or a cordless phone, a communication radio wave in the frequency band (carrier frequency) dedicated to each type of communication is conventionally used. Therefore, the front ends of the receiver and the transmitter included in a portable phone are provided with a high-frequency amplifier for amplifying only high-frequency signals in a single frequency band specified for a communication radio wave.
A high-frequency amplifier according to a conventional technology will be described below by referring to FIG. 18 to FIG. 24.
With a block diagram shown in FIG. 18, a high-frequency amplifier according to a conventional technology will be described first. A high-frequency amplifier 1 is roughly formed of an amplifying circuit 7 described later, a single frequency matching circuit (input matching circuit) 4 connected between the input of the amplifying circuit 7 and an input terminal 2, and a single-frequency matching circuit (output matching circuit) 12 connected between the output of the amplifying circuit 7 and an output terminal 16.
The high-frequency amplifier 1 is provided for the receiver of a portable telephone handling a communication radio wave in the frequency band ranging from 1.8 GHz to 2.0 GHz. The configuration of the high-frequency amplifier 1 will be described below in detail by referring to FIG. 19.
The input terminal 2 of the high-frequency amplifier 1 is connected to, for example, an antenna of a portable telephone. When a communication radio wave is received, a high-frequency signal in the frequency band ranging from 1.8 GHz to 2.0 GHz is input to the input terminal 2.
A capacitor 3 is connected directly to the input terminal 2, and removes a DC component when the DC component is included in a high-frequency signal input to the input terminal 2. The capacitor 3 is set to have a capacitance which does not adversely affect a high-frequency signal input to the input terminal 2, that is, for example, set to about 20 pF.
An input matching circuit 4 for matching a high-frequency signal in the frequency band ranging from 1.8 GHz to 2.0 GHz includes a coil 5 connected between the capacitor 3 and the gate terminal G1 of an input transistor 8 described later, and a coil 6 connected between ground and the connection point E between the capacitor 3 and the coil 5.
The inductances of the coil 5 and the coil 6 are set such that input matching is performed for a high-frequency signal in the frequency band ranging from 1.8 GHz to 2.0 GHz. The inductance of the coil 5 is set to 12 nH, for example, and the inductance of the coil 6 is set to 10 nH.
An amplifying circuit 7 is connected to the output of the input matching circuit 4. The amplifying circuit 7 includes the input transistor 8, an output transistor 9 connected in cascade to the input transistor 8, a resistor 10, and a bypass capacitor 11. Field effect transistors (FETs) are used for the input transistor 8 and the output transistor 9. The resistor 10 and the bypass capacitor 11 form a self-bias circuit for applying a DC bias to a high-frequency signal which is input to the amplifying circuit 7. The resistance of the resistor 10 is set to 80 .OMEGA., for example, and the capacitance of the bypass capacitor 11 is set to 100 pF.
An output matching circuit 12 for matching a high-frequency signal in the frequency band ranging from 1.8 GHz to 2.0 GHz is connected to the output of the amplifying circuit 7. The output matching circuit 12 includes a coil 13 connected between the drain terminal D2 of the output transistor 9 in the amplifying circuit 7 and a capacitor 15 described later, and a coil 14 connected at one end to the connection point F between the coil 13 and the capacitor 15. The other end of the coil 14 is connected to a power supply for supplying a fixed voltage Vd.
The inductances of the coil 13 and the coil 14 are set such that output matching is performed for a high-frequency signal in the frequency band ranging from 1.8 GHz to 2.0 GHz. The inductance of the coil 13 is set to 8.2 nH, for example, and the inductance of the coil 14 is set to 10 nH.
The input matching circuit 4 and the output matching circuit 12 have the structure of a single-frequency matching circuit for matching a single frequency.
A capacitor 15 is connected to the output of the output matching circuit 12, and removes a DC bias applied to a high-frequency signal when the high-frequency signal is amplified by the amplifying circuit 7. The capacitance of the capacitor 15 is set to a capacitance which does not adversely affect a high-frequency signal output from the output terminal 16, that is, for example, set to about 20 pF.
The output terminal 16 of the high-frequency amplifier 1 is provided outside the high-frequency amplifier 1 and connected to a signal processing circuit (not shown) for handling modulation and demodulation, or audio information.
The high-frequency amplifier according to the conventional technology has the above structure. The high-frequency amplifier 1 is configured such that it matches and amplifies only a high-frequency signal in the frequency band ranging from 1.8 GHz to 2.0 GHz. In other words, the high-frequency amplifier 1 is configured such that it reduces the noise factor as much as possible, increases the gain as much as possible, and reduces the input reflection coefficient and the output reflection coefficient as much as possible for a high-frequency signal in the frequency band ranging from 1.8 GHz to 2.0 GHz.
In FIG. 20, a characteristic curve "a" indicates the frequency characteristic of the noise factor in the high-frequency amplifier 1. In FIG. 21, a characteristic curve "b" indicates the frequency characteristic of the gain in the high-frequency amplifier 1. In FIG. 22, characteristic curves "c" and "d" indicate the frequency characteristics of the input reflection coefficient and the output reflection coefficient in the high-frequency amplifier 1. It is found from FIG. 20 to FIG. 22 that, at the frequency band ranging from 1.8 GHz to 2.0 GHz, the noise factor is minimum, the gain is maximum, and the input reflection coefficient and the output reflection coefficient are minimum.
As described above, the high-frequency amplifier 1 according to the conventional technology is configured such that it achieves a superior amplification effect only on a high-frequency signal in the frequency band ranging from 1.8 GHz to 2.0 GHz.
In another known high-frequency amplifier 1' shown in FIG. 23, a high-frequency amplifier in which the resistor 10 and the bypass capacitor 11 are omitted from the amplifying circuit 7 and one end of the coil 6 is not connected to ground but instead is connected to a fixed voltage Vd' of, for example, -0.7 V. This type of conventional technology also achieves a superior amplification effect on a high-frequency signal in the frequency band ranging from 1.8 GHz to 2.0 GHz.
The respective frequency bands of communication radio waves that are used in different areas may differ. For example, the frequency band of a communication radio wave used in one area may be 1.8 GHz to 2.0 GHz whereas that used in another area may be 0.7 GHz to 1.0 GHz.
The high-frequency amplifier 1 according to the conventional technology described above amplifies only a high-frequency signal in a single frequency band. Therefore, to implement a portable telephone which handles a plurality of communication radio waves in different frequency bands (that is, a common portable telephone usable in a plurality of areas), the portable telephone needs to be equipped with a plurality of high-frequency amplifiers 1 each corresponding to a respective frequency band. As a result, the portable telephone becomes large, power consumption increases, and cost also increases.
Since the high-frequency amplifier 1 according to the conventional technology is used in a mobile communication unit, the characteristic curve of input power level vs. output power level needs to provide low distortion and high sound quality. To this end, an intercept point P, which indicates the quality of the characteristic of input power level vs. output power level, must be good, as will be discussed further below.
The intercept point P will be described below. A single-frequency signal is input to the input end of the high-frequency amplifier 1 used in a conventional mobile communication unit. The single-frequency signal actually includes, however, a plurality of high-frequency signals within a very narrow band, such as a 1.9003-GHz signal, a 1.9006-GHz signal, and a 1.9009-GHz signal, used as information carriers. Thus, in effect, a plurality of high-frequency signals are input to the high-frequency amplifier 1.
When two adjacent high-frequency signals, such as a 1.9003-GHz signal and a 1.9006-GHz signal, having the same input power level, are input to the high-frequency amplifier 1, for example, the high-frequency amplifier 1 outputs the fundamental-wave signals having the same waveform corresponding to the two high-frequency signals amplified by the high-frequency amplifier 1, and also outputs third intermodulation wave signals having a distorted waveform caused by mixing the two high-frequency signals in the high-frequency amplifier 1.
As shown in FIG. 24, the horizontal axis represents an input power level and the vertical axis represents the output power levels of the fundamental-wave signal and the third intermodulation waveform-distorted wave signal corresponding to an input power level. A characteristic curve "e" for the fundamental-wave signal indicates the relationship between the input power level and the output power level of the fundamental-wave signal. A characteristic curve "f" for the third intermodulation waveform-distorted wave signal indicates the relationship between the input power level and the output power level of the third intermodulation signal. The characteristic curves "e" and "f" show linearity in a zone A where the input power level is low, and are distorted and show saturation in a zone B where the input power level is high.
The intercept point P is an intersection obtained by extrapolating the straight sections of the characteristic curves "e" and "f" in the zone A, where the input power level is low. A good intercept point P means that the input power level (hereinafter called input intercept point) and the output power level (hereinafter called output intercept point) specified by the intercept point P are large. In other words, a good intercept point P means that the characteristic curves "e" and "f" of the high-frequency amplifier 1 are not distorted in the zone B, where the input power level is high, and have good linearity.
In the high-frequency amplifier 1 according to the conventional technology, the values of the components in the output matching circuit 12 are set in order to achieve impedance matching for a high-frequency signal output from the output terminal 16. The values of the components in the output matching circuit 12 are also specified in order to improve the intercept point P. However, it is difficult to satisfy both characteristics at the same time just by setting the values of the components in the output matching circuit 12, and the stability of the high-frequency amplifier 1 can be decreased.